Thrust efficient turbofan engine

ABSTRACT

A turbofan engine includes a gas generator section for generating a gas stream flow with higher energy per unit mass flow than that contained in the ambient air and a power turbine that converts the gas stream flow into shaft power. The turbofan engine further includes a propulsor section including a fan driven by the power turbine through a geared architecture at a second speed lower than the first speed for generating propulsive thrust as a mass flow rate of air through a bypass flow path. An Engine Unit Thrust Parameter defined as net engine thrust divided by a product of the mass flow rate of air through the bypass flow path, a tip diameter of the fan and the first rotational speed of the power turbine is less than about 0.15 at a take-off condition.

REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application No. 61/787,321 filed on Mar. 15, 2013.

BACKGROUND

A turbofan engine typically includes a fan section, a compressor section, a combustor section and a turbine section. Air entering the compressor section is typically compressed and delivered into the combustor section where it is mixed with fuel and ignited to generate a high-pressure, high-temperature gas flow. The high-pressure, high-temperature gas flow expands through the turbine section to drive the compressor and the fan section.

A direct-drive turbofan engine typically includes a fan section directly driven by a low pressure turbine producing the power needed to drive the fan section, such that the low pressure turbine and the fan section rotate at a common rotational speed in a common direction. A power transmission device such as a gear assembly or other mechanism may be utilized to drive the fan section such that the fan section may rotate at a speed different than the turbine section so as to increase the overall efficiency of the engine. In a gear-drive turbofan engine architecture, a shaft driven by one of the turbine sections may provide an input to the speed reduction device that drives the fan section at a reduced speed such that both the turbine section and the fan section can rotate at closer to their respective optimal rotational speeds.

SUMMARY

A turbofan engine according to an exemplary embodiment of this disclosure, among other possible things includes a gas generator section for generating a gas stream flow with higher energy per unit mass flow than that contained in ambient air, a power turbine converting the gas stream flow into shaft power, the power turbine rotating at a first rotational speed, a speed reduction device driven by the power turbine, and a propulsor section including a fan driven by the power turbine through the speed reduction device at a second speed lower than the first speed for generating propulsive thrust as a mass flow rate of air through a bypass flow path, wherein an Engine Unit Thrust Parameter (“EUTP”) defined as net engine thrust divided by a product of the mass flow rate of air through the bypass flow path, a tip diameter of the fan and the first rotational speed of the power turbine is less than about 0.15 at a take-off condition.

In a further embodiment of the foregoing turbofan engine the EUTP at one of a climb condition and a cruise condition is less than about 0.10.

In a further embodiment of any of the foregoing turbofan engine embodiments, the EUTP at one of a climb condition and a cruise condition is less than about 0.08.

In a further embodiment of any of the foregoing turbofan engine embodiments, wherein the tip diameter of the fan is between about 50 inches and about 160 inches.

In a further embodiment of any of the foregoing turbofan engine embodiments, the mass flow generated by the propulsor section is between about 625 lbm/hour and about 80,000 lbm/hour.

In a further embodiment of any of the foregoing turbofan engine embodiments, the first speed of the power turbine is between about 6200 rpm and about 12,500 rpm.

In a further embodiment of any of the foregoing turbofan engine embodiments, the propulsive thrust generated by the turbofan engine is between about 16,000 lbf and about 120,000 lbf.

In a further embodiment of any of the foregoing turbofan engine embodiments, the gas generator defines an overall pressure ratio of between about 40 and about 80.

Another turbofan engine according to an exemplary embodiment of this disclosure, among other possible things includes a gas generator section for generating a gas stream flow with higher energy per unit mass flow than that contained in ambient air, a power turbine converting the gas stream flow into shaft power, the power turbine rotating at a first rotational speed, a speed reduction device driven by the power turbine, and a propulsor section including a fan driven by the power turbine through the speed reduction device at a second speed lower than the first speed for generating propulsive thrust as a mass flow rate of air through a bypass flow path, wherein an Engine Unit Thrust Parameter (“EUTP”) defined as net engine thrust divided by a product of the mass flow rate of air through the bypass flow path, a tip diameter of the fan and the first rotational speed of the power turbine is less than about 0.10 at one of a climb condition and a cruise condition.

In a further embodiment of the foregoing turbofan engine the EUTP at one of the climb condition and the cruise condition is less than about 0.08.

In a further embodiment of any of the foregoing turbofan engine embodiments, the EUTP at a take-off condition is less than about 0.15.

In a further embodiment of any of the foregoing turbofan engine embodiments, the tip diameter of the fan is between about 50 inches and about 160 inches.

In a further embodiment of any of the foregoing turbofan engine embodiments, the mass flow generated by the propulsor section is between about 625 lbm/hour and about 80,000 lbm/hour.

In a further embodiment of any of the foregoing turbofan engine embodiments, the first speed of the power turbine is between about 6200 rpm and about 12,500 rpm.

In a further embodiment of any of the foregoing turbofan engine embodiments, the propulsive thrust generated by the turbofan engine is between about 16,000 lbf and about 120,000 lbf.

In a further embodiment of any of the foregoing turbofan engine embodiments, the gas generator defines an overall pressure ratio of between about 40 and about 80.

Another turbofan engine according to an exemplary embodiment of this disclosure, among other possible things includes a gas generator section for generating a high energy gas stream, the gas generating section including a compressor section, combustor section and a first turbine, a second turbine converting the high energy gas stream flow into shaft power, the second turbine rotating at a first speed and including less than or equal to about six (6) stages, a geared architecture driven by the second turbine, and a propulsor section driven by the second turbine through the geared architecture at a second speed lower than the first speed, the propulsor section including a fan with a pressure ratio across the fan section less than about 1.45, the propulsor section generating propulsive thrust as a mass flow rate of air through a bypass flow path from the shaft power, wherein an Engine Unit Thrust Parameter (“EUTP”) defined as net engine thrust divided by a product of a mass flow rate of air through the bypass flow path, a tip diameter of the fan and the first rotational speed of the second turbine is less than about 0.15 at a take-off condition.

In a further embodiment of the foregoing turbofan engine, the EUTP at one of a climb condition and a cruise condition is less than about 0.10.

In a further embodiment of any of the foregoing turbofan engine embodiments, the EUTP at the take-off condition is less than about 0.08.

In a further embodiment of any of the foregoing turbofan engine embodiments, the fan section defines a bypass airflow having a bypass ratio greater than about ten (10).

In a further embodiment of any of the foregoing turbofan engine embodiments, the tip diameter of the fan is between about 50 inches and about 160 inches.

In a further embodiment of any of the foregoing turbofan engine embodiments, the mass flow generated by the propulsor section is between about 625 lbm/hour and about 80,000 lbm/hour.

In a further embodiment of any of the foregoing turbofan engine embodiments, the first speed of the second turbine is between about 6200 rpm and about 12,500 rpm.

In a further embodiment of any of the foregoing turbofan engine embodiments, the second turbine comprises a low pressure turbine with 3 to 6 stages.

Although the different examples have the specific components shown in the illustrations, embodiments of this disclosure are not limited to those particular combinations. It is possible to use some of the components or features from one of the examples in combination with features or components from another one of the examples.

These and other features disclosed herein can be best understood from the following specification and drawings, the following of which is a brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an example turbofan engine.

FIG. 2 is a schematic view of functional elements of the example turbofan engine.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates an example gas turbine engine 20 that includes a fan section 22, a compressor section 24, a combustor section 26 and a turbine section 28. Alternative engines might include an augmenter section (not shown) among other systems or features. The fan section 22 drives air through a bypass flow path B while the compressor section 24 draws air in along a core flow path C where air is compressed and communicated to a combustor section 26. In the combustor section 26, air is mixed with fuel and ignited to generate a high pressure exhaust gas stream that expands through the turbine section 28 where energy is extracted and utilized to drive the fan section 22 and the compressor section 24.

Although the disclosed non-limiting embodiment depicts a two-spool turbofan gas turbine engine, it should be understood that the concepts described herein are not limited to use with two-spool turbofans as the teachings may be applied to other types of turbine engines; for example a turbine engine including a three-spool architecture in which three spools concentrically rotate about a common axis and where a low spool enables a low pressure turbine to drive a fan via a speed reduction device such as a gearbox, an intermediate spool that enables an intermediate pressure turbine to drive a first compressor of the compressor section, and a high spool that enables a high pressure turbine to drive a high pressure compressor of the compressor section.

The example engine 20 generally includes a low speed spool 30 and a high speed spool 32 mounted for rotation about an engine central longitudinal axis A relative to an engine static structure 36 via several bearing systems 38. It should be understood that various bearing systems 38 at various locations may alternatively or additionally be provided and that the location of the bearing systems 38 may be varied as appropriate to the application.

The low speed spool 30 generally includes an inner shaft 40 that connects a fan 42 and a low pressure (or first) compressor section 44 to a low pressure (or first) turbine section 46. The inner shaft 40 drives the fan 42 through a speed change device, such as a geared architecture 48, to drive the fan 42 at a lower speed than the low speed spool 30. The high-speed spool 32 includes an outer shaft 50 that interconnects a high pressure (or second) compressor section 52 and a high pressure (or second) turbine section 54. The inner shaft 40 and the outer shaft 50 are concentric and rotate via the bearing systems 38 about the engine central longitudinal axis A.

A combustor 56 is arranged between the high pressure compressor 52 and the high pressure turbine 54. In one example, the high pressure turbine 54 includes at least two stages to provide a double stage high pressure turbine 54. In another example, the high pressure turbine 54 includes only a single stage. As used herein, a “high pressure” compressor or turbine experiences a higher pressure than a corresponding “low pressure” compressor or turbine. It will be appreciated that each of the positions of the fan section 22, compressor section 24, combustor section 26, turbine section 28, and fan drive gear system 48 may be varied. For example, gear system 48 may be located aft of combustor section 26 or even aft of turbine section 28, and fan section 22 may be positioned forward or aft of the location of gear system 48.

The example low pressure turbine 46 has a pressure ratio that is greater than about five (5). The pressure ratio of the example low pressure turbine 46 is measured prior to an inlet of the low pressure turbine 46 as related to the pressure measured at the outlet of the low pressure turbine 46 prior to an exhaust nozzle.

A mid-turbine frame 58 of the engine static structure 36 is arranged generally between the high pressure turbine 54 and the low pressure turbine 46. The mid-turbine frame 58 further supports bearing systems 38 in the turbine section 28 as well as setting airflow entering the low pressure turbine 46.

Airflow through the core airflow path C is compressed by the low pressure compressor 44 then by the high pressure compressor 52 mixed with fuel and ignited in the combustor 56 to produce a gas stream with high energy that expands through the high pressure turbine 54 and low pressure turbine 46. The mid-turbine frame 58 includes vanes 60, which are in the core airflow path C and function as an inlet guide vane for the low pressure turbine 46. Utilizing the vane 60 of the mid-turbine frame 58 as the inlet guide vane for low pressure turbine 46 decreases the length of the low pressure turbine 46 without increasing the axial length of the mid-turbine frame 58. Reducing or eliminating the number of vanes in the low pressure turbine 46 shortens the axial length of the turbine section 28. Thus, the compactness of the gas turbine engine 20 is increased and a higher power density may be achieved.

The disclosed gas turbine engine 20 in one example is a high-bypass geared aircraft engine. In a further example, the gas turbine engine 20 includes a bypass ratio greater than about six (6), with an example embodiment being greater than about ten (10). The example speed reduction device is a geared architecture 48 however other speed reducing devices such as fluid or electromechanical devices are also within the contemplation of this disclosure. The example geared architecture 48 is an epicyclical gear train, such as a planetary gear system, star gear system or other known gear system, with a gear reduction ratio of greater than about 1.8 and, in some embodiments, greater than about 4.5.

In one disclosed embodiment, the gas turbine engine 20 includes a bypass ratio greater than about ten (10:1) and the fan diameter is significantly larger than an outer diameter of the low pressure compressor 44. It should be understood, however, that the above parameters are only exemplary of one embodiment of a gas turbine engine including a geared architecture and that the present disclosure is applicable to other gas turbine engines.

A significant amount of thrust is provided by airflow through the bypass flow path B due to the high bypass ratio. The fan section 22 of the engine 20 is designed for a particular flight condition—typically cruise at about 0.8 Mach and about 35,000 feet. The flight condition of 0.8 Mach and 35,000 ft., with the engine at its best fuel consumption—also known as “bucket cruise Thrust Specific Fuel Consumption (‘TSFC’)”—is the industry standard parameter of pound-mass (lbm) of fuel per hour being burned divided by pound-force (lbf) of thrust the engine produces at that minimum point.

“Low fan pressure ratio” is the pressure ratio across the fan blade alone, without a Fan Exit Guide Vane (“FEGV”) system. The low fan pressure ratio as disclosed herein according to one non-limiting embodiment is less than about 1.50. In another non-limiting embodiment the low fan pressure ratio is less than about 1.45.

“Low corrected fan tip speed” is the actual fan tip speed in ft/sec divided by an industry standard temperature correction of [(Tram ° R)/(518.7° R)]^(0.5). The “Low corrected fan tip speed”, as disclosed herein according to one non-limiting embodiment, is less than about 1150 ft/second.

The example gas turbine engine includes the fan 42 that comprises in one non-limiting embodiment less than about twenty-six (26) fan blades. In another non-limiting embodiment, the fan section 22 includes less than about twenty (20) fan blades. Moreover, in one disclosed embodiment the low pressure turbine 46 includes no more than about six (6) stages schematically indicated at 34. In another non-limiting example embodiment the low pressure turbine 46 includes about three (3) stages. A ratio between the number of fan blades 42 and the number of low pressure turbine rotors is between about 3.3 and about 8.6. The example low pressure turbine 46 provides the driving power to rotate the fan section 22 and therefore the relationship between the number of stages 34 in the low pressure turbine 46 and the number of blades 42 in the fan section 22 defines an example gas turbine engine 20 with increased power transfer efficiency.

Referring to FIG. 2, with continued reference to FIG. 1, the example turbofan engine 20 includes a gas generator section 62 for generating a high energy (per unit mass) gas stream 78. A power turbine 76 converts the high energy gas stream 78 into shaft power that drives the geared architecture 48. In one embodiment, the power turbine may be the low pressure turbine 46 that drives the inner shaft 40. The power turbine 76 drives a propulsor section 64 through the geared architecture 48. The propulsor section 64 generates a mass flow 70 of air through the bypass flow path B that is a substantial portion of the overall propulsive thrust 68 generated by the turbofan engine 20.

The gas generator section 62 includes part of the fan section compressing the air flow directed along the core flow path C, the low pressure compressor 44, the high pressure compressor 52, the combustor 56, the high pressure turbine 54 and part of the low-pressure turbine 46. The high pressure turbine 54 is the first turbine after the combustor 56 and drives the high pressure compressor 52. Disclosed example gas generators include an overall pressure ratio between entering airflow and the exiting gas stream of between about 40 and 80.

The power driving the geared architecture 48 and thereby the propulsor section 64 is provided by the power turbine 76. In this disclosure, the power turbine 76 comprises the second turbine downstream of the combustor 56 such as the low pressure turbine 46. The low pressure turbine 46 rotates at the first speed 72 and includes no more than six (6) stages 34. Moreover, the low pressure turbine 46 may include between three (3) and six (6) stages 34, inclusive.

The power or low pressure turbine 46 rotates at the first speed 72 (measured in terms of revolutions per minute) greater than a second speed 74 (also measured in terms of revolutions per minute) at which the fan section 22 rotates. In some embodiments, the first speed may be between about 6200 rpm and about 12,500 rpm. The first speed of the low pressure turbine 46 is enabled by the speed reduction provided by the geared architecture 48. At the first speed 72, each of the stages 34 are more efficient at converting energy from the gas flow 78 to power transmitted through the inner shaft 40.

The propulsor section 64 includes the fan section with fan blades 42 that rotate about the engine axis A. The fan blades 42 extend radially outward to define a tip diameter 66 between tips of opposing blades 42. The disclosed fan section 22 includes a tip diameter 66 that, in some embodiments, may be between about 45 inches (114 cm) and about 160 inches (406 cm). In another example embodiment, the tip diameter 66 is between about 50 inches (127 cm) and about 85 inches (215.9 cm). The tip diameter of the fan section 22 enables the desired fan pressure ratio in combination with the second rotational speed 74 provided by the gear reduction of the geared architecture 48.

The propulsor section 64 includes the fan section 22, and also includes the fan exit guide vanes 80 and typically a fan nozzle 82. The fan section 22 is rotated at the second speed 74 by the geared architecture 48 at a speed determined to enable the generation of the mass flow 70 through the bypass flow path B. The pressure ratio across the fan section enables the efficient transformation of the shaft power provided in the power turbine 76 to propulsive thrust.

Fan pressure ratios below about 1.5, and better below 1.45 enable desirable generation of thrust. The desired fan pressure ratio can be obtained utilizing a combination of fan exit guide vanes 80 and the fan nozzle 82 that cooperate with the fan section 22 to enable fan pressure ratios of less than 1.45. The mass flow 70 produced by the example propulsor section 64 may, in some embodiments, be between about 625 lbm/hour (283 kg/hour) and about 80,000 lbm/hour (36,287 kg/hour). The mass flow of air 70 through the bypass flow path B combines with thrust generated by gas flow 78 to provide the overall engine thrust 68. However, a majority of engine thrust is provided by the mass flow of air 70 generated by the propulsor section 64.

The overall efficiency of the turbofan engine 20 is a combination of how well each of the sections 62, 76 and 64 converts input energy into the desired output. The gas generator section 62 transforms energy from the air/fuel mixture ignited in the combustor 56 into the high-energy gas stream 78. The power turbine 46 converts energy from the gas stream 78 into shaft power rotating the inner shaft 40 at a first speed 72 to drive the propulsor 64. The propulsor section 64 generates the mass flow of air 70 through bypass flow path B that provides the propulsive thrust 68.

The thrust generation efficiency of the engine is related to the Engine Unit Thrust Parameter (“EUTP”), which is defined as the net thrust produced by the engine divided by the product of mass flow rate of air through the fan bypass section, the fan tip diameter and the rotational speed of the power turbine section, as set out in Equation 1.

$\begin{matrix} {{{Engine}\mspace{14mu}{Unit}\mspace{14mu}{Thrust}\mspace{14mu}{Parameter}} = \frac{{Net}\mspace{14mu}{Thrust}\mspace{14mu}{of}\mspace{14mu}{the}\mspace{14mu}{Engine}}{\begin{bmatrix} \left( {{mass}\mspace{14mu}{flow}\mspace{14mu}{rate}\mspace{14mu}{of}\mspace{14mu}{air}\mspace{14mu}{through}\mspace{14mu}{fan}\mspace{14mu}{bypass}} \right) \\ \left( {{Fan}\mspace{14mu}{Tip}\mspace{14mu}{Diameter}} \right) \\ \left( {{Speed}\mspace{14mu}{of}{\mspace{11mu}\;}{the}\mspace{14mu}{power}\mspace{14mu}{turbine}} \right) \end{bmatrix}}} & {{Equation}\mspace{14mu} 1} \end{matrix}$

The EUTP is a dimensionless quantity calculated utilizing the net engine thrust, the mass flow rate of air, the tip diameter and the power turbine rotational speed expressed in appropriate units. For example, if the SI system of units is used, the units for these four quantities will be N, kg/s, m and radians/s, respectively. The calculation of the EUTP will be straight forward with no need to use conversion factors. If a “customary” set of units are used, i.e., engine thrust expressed in lbf, mass flow rate expressed in lbm/s, fan diameter express in inches and rotational speed expressed in RPM, then the ratio calculated using these values are multiplied by constant approximately equal to 3686.87 to account for all conversion factors necessary to get all parameters in self-consistent units.

Embodiments of the geared gas turbine engine 20 including the disclosed features and configurations produce thrust ranging between about 16,000 lbf (71,171 N) and about 120,000 lbf (533,786 N). The EUTP for the disclosed turbofan engine 20 is less than those provided in prior art turbine engines. Three disclosed exemplary engines which incorporate power turbine and propulsor sections as set forth in this application are described and contrasted with prior art engine examples in Table 1.

TABLE 1 Engine 1 Engine 2 Engine 3 Prior Art Engine 1 Prior Art Engine 2 Fan Diameter in 55.9 73.0 81.0 63.5 49.2 Thrust Class lbf 17K 23.3K 33K 33K 21K Max Climb Thrust lbf 3526 4878 6208 9721 8587 Fan face corr. Flow lbm/sec 703.4 1212.1 1512.4 847.0 502.6 Fan OD corr. Flow lbm/sec 626.3 1108.8 1388.6 696.4 314.2 Fan face physical flow lbm/sec 261.5 450.6 561.7 519.4 308.8 Fan OD physical flow lbm/sec 232.7 412.1 515.6 426.9 193.1 Second speed (fan) RPM 4913 3377 3099 4969 7640 First speed (power turbine) RPM 11835 10341 9491 4969 7640 Engine Unit Thrust Parameter 0.08 0.06 0.06 0.27 0.44 Average Cruise Thrust lbf 2821 3929 4729 5300 4141 Fan face corr. Flow lbm/sec 668.3 1157.6 1429.4 845.6 490.5 Fan OD corr. Flow lbm/sec 598.0 1065.6 1322.6 695.5 312.1 Fan face physical flow lbm/sec 254.1 440.2 543.4 327.4 181.1 Fan OD physical flow lbm/sec 227.3 405.2 502.7 269.2 115.2 Second speed (fan) RPM 4472 3070 2748 4769 6913 First speed (power turbine) RPM 10774 9402 8416 4769 6913 Engine Unit Thrust Parameter 0.08 0.05 0.05 0.24 0.39 Max Takeoff Thrust lbf 12500 18735 25678 25382 17941 Fan face corr. Flow lbm/sec 610.0 1032.5 1438.8 871.1 496.6 Fan OD corr. Flow lbm/sec 546.6 948.5 1330.2 711.8 312.1 Fan face physical flow lbm/sec 611.0 1029.4 1452.2 901.5 509.4 Fan OD physical flow lbm/sec 547.4 945.6 1342.4 736.1 320.2 Second speed (fan) RPM 4689 3249 3117 5411 7791 First speed (power turbine) RPM 11295 9951 9546 5411 7791 Engine Unit Thrust Parameter 0.13 0.10 0.09 0.37 0.54

In some example embodiments, the EUTP is as low as 0.05 at a cruise condition and less than about 0.10 at maximum takeoff thrust. In further embodiments, the EUTP is less than about 0.08 at a cruise and/or a climb condition.

Moreover, the EUTP is accomplished through the use of the geared architecture 48 at a gear ratio of, in some embodiments, greater than about 2.3. In other embodiments, the gear ratio may be greater than about 2.8. Accordingly, a ratio of the EUTP to the gear ratio further defines physical operating characteristics of the disclosed engines. In one disclosed embodiment, a ratio of the EUTP at takeoff to a gear ratio of 2.8 is about 0.028. In another disclosed ratio of the EUTP at a climb or cruise condition to the gear ratio of 2.8 is between about 0.036 and 0.054.

Accordingly, the EUTP for engines based upon the disclosed features may be less than 0.15 when the engine is operating at take-off condition, while it may have a value less than 0.1 when operating at climb and cruise conditions.

Accordingly, the disclosed embodiments of each of the gas generator 62, power turbine 76 and propulsor sections 64 of the geared engine embodiments efficiently convert energy to provide a more thrust efficient turbofan engine as compared to conventional non-geared engines.

Although various example embodiments have been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this disclosure. For that reason, the following claims should be studied to determine the scope and content of this disclosure. 

What is claimed is:
 1. A turbofan engine comprising: a gas generator section for generating a gas stream flow with higher energy per unit mass flow than that contained in ambient air; a power turbine converting the gas stream flow into shaft power, the power turbine rotating at a first rotational speed; a speed reduction device driven by the power turbine; and a propulsor section including a fan driven by the power turbine through the speed reduction device at a second speed lower than the first speed for generating propulsive thrust as a mass flow rate of air through a bypass flow path, wherein an Engine Unit Thrust Parameter (“EUTP”) defined as net engine thrust divided by a product of the mass flow rate of air through the bypass flow path, a tip diameter of the fan and the first rotational speed of the power turbine is less than about 0.15 at a take-off condition.
 2. The turbofan engine as recited in claim 1, wherein the EUTP at one of a climb condition and a cruise condition is less than about 0.10.
 3. The turbofan engine as recited in claim 1, wherein the EUTP at one of a climb condition and a cruise condition is less than about 0.08.
 4. The turbofan engine as recited in claim 1, wherein the tip diameter of the fan is between about 50 inches and about 160 inches.
 5. The turbofan engine as recited in claim 1, wherein the mass flow generated by the propulsor section is between about 625 lbm/hour and about 80,000 lbm/hour.
 6. The turbofan engine as recited in claim 1, wherein the first speed of the power turbine is between about 6200 rpm and about 12,500 rpm.
 7. The turbofan engine as recited in claim 1, wherein the propulsive thrust generated by the turbofan engine is between about 16,000 lbf and about 120,000 lbf.
 8. The turbofan engine as recited in claim 7, wherein the gas generator defines an overall pressure ratio of between about 40 and about
 80. 9. A turbofan engine comprising: a gas generator section for generating a gas stream flow with higher energy per unit mass flow than that contained in ambient air; a power turbine converting the gas stream flow into shaft power, the power turbine rotating at a first rotational speed; a speed reduction device driven by the power turbine; and a propulsor section including a fan driven by the power turbine through the speed reduction device at a second speed lower than the first speed for generating propulsive thrust as a mass flow rate of air through a bypass flow path, wherein an Engine Unit Thrust Parameter (“EUTP”) defined as net engine thrust divided by a product of the mass flow rate of air through the bypass flow path, a tip diameter of the fan and the first rotational speed of the power turbine is less than about 0.10 at one of a climb condition and a cruise condition.
 10. The turbofan engine as recited in claim 9, wherein the EUTP at one of the climb condition and the cruise condition is less than about 0.08.
 11. The turbofan engine as recited in claim 9, wherein the EUTP at a take-off condition is less than about 0.15.
 12. The turbofan engine as recited in claim 9, wherein the tip diameter of the fan is between about 50 inches and about 160 inches.
 13. The turbofan engine as recited in claim 9, wherein the mass flow generated by the propulsor section is between about 625 lbm/hour and about 80,000 lbm/hour.
 14. The turbofan engine as recited in claim 1, wherein the first speed of the power turbine is between about 6200 rpm and about 12,500 rpm.
 15. The turbofan engine as recited in claim 9, wherein the propulsive thrust generated by the turbofan engine is between about 16,000 lbf and about 120,000 lbf.
 16. The turbofan engine as recited in claim 9, wherein the gas generator defines an overall pressure ratio of between about 40 and about
 80. 17. A turbofan engine comprising: a gas generator section for generating a high energy gas stream, the gas generating section including a compressor section, combustor section and a first turbine; a second turbine converting the high energy gas stream flow into shaft power, the second turbine rotating at a first speed and including less than or equal to about six (6) stages; a geared architecture driven by the second turbine; and a propulsor section driven by the second turbine through the geared architecture at a second speed lower than the first speed, the propulsor section including a fan with a pressure ratio across the fan section less than about 1.45, the propulsor section generating propulsive thrust as a mass flow rate of air through a bypass flow path from the shaft power, wherein an Engine Unit Thrust Parameter (“EUTP”) defined as net engine thrust divided by a product of a mass flow rate of air through the bypass flow path, a tip diameter of the fan and the first rotational speed of the second turbine is less than about 0.15 at a take-off condition.
 18. The turbofan engine as recited in claim 17, wherein the EUTP at one of a climb condition and a cruise condition is less than about 0.10.
 19. The turbofan engine as recited in claim 17, wherein the EUTP at the take-off condition is less than about 0.08.
 20. The gas turbofan engine as recited in claim 17, wherein the fan section defines a bypass airflow having a bypass ratio greater than about ten (10).
 21. The turbofan engine as recited in claim 17, wherein the tip diameter of the fan is between about 50 inches and about 160 inches.
 22. The turbofan engine as recited in claim 17, wherein the mass flow generated by the propulsor section is between about 625 lbm/hour and about 80,000 lbm/hour.
 23. The turbofan engine as recited in claim 17, wherein the first speed of the second turbine is between about 6200 rpm and about 12,500 rpm.
 24. The turbofan engine as recited in claim 17, wherein the second turbine comprises a low pressure turbine with 3 to 6 stages. 